1. The Field of the Invention
The present invention relates to correction of an aberration of a light beam used for reading and writing bits of information from and onto a recording medium.
2. Description of the Related Art
In a pickup placed in front of an optical disk, a liquid crystal layer has been used as one of conventional countermeasures for correcting aberration of a light beam, as disclosed by Japanese Patent Laid-open (KOKAI) No. 9 (1997)-128785.
This correcting technique is illustrated in FIGS. 1A and 1B. As shown in FIG. 1A, a liquid crystal layer 2 is placed in front of an objective lens 1 in order to obtain a high-quality spot on an optical disk DK1. The liquid crystal layer 2 is in charge of generating an appropriate optical phase distribution for canceling an aberration of light caused by both the objective lens d and the disk DK1. Since the liquid crystal layer 2 becomes active when a linearly polarized light in a particular direction enters the layer 2, the incident light should be linearly polarized in the particular direction (in this explanation, such linearly polarized light is referred to as s-polarized light). To obtain a linearly polarized light spot on the optical disk DK 1, the optical configuration shown in FIG. 1A is provided.
By contrast, if a circularly polarized spot on an optical disk is required, the optical configuration shown in FIG. 1B is provided, in which a quarter wave plate 6 to convert linearly polarized light to circularly polarized light is arranged additionally to the configuration shown in FIG. 1A.
In this optical configuration shown in 1B, the returning light from an optical disk DK2 has a polarizing direction perpendicular to that of the incident light (such polarized light is referred to as p-polarized light). To make the liquid crystal layer operative as aberration correcting means in the outward and homeward optical paths, it is required, as shown in FIG. 1B, two liquid crystal layers consisting of a liquid crystal layer 4 for s-polarized light and a further crystal layer 5 for s-polarized light. Only a difference between the two liquid crystal layers 4 and 5 is the direction of orientation of liquid crystal molecules. That is, the direction of orientation of liquid crystal molecules within one liquid crystal layer 4 is perpendicular to that within the other liquid crystal layer 5. However, the operations of both the liquid crystal layers 4 and 5 are almost similar to each other, thus being in charge of creating an approximately identical phase distribution.
For a DVD (Digital Versatile Disc) and a next-generation high-density disk, it has been recognized that the circularly polarized optical spot on a disk is desired to obtain an excellent playablity. Hence if such a optical spot is desired, it is required to choose the optical configuration shown in FIG. 1B.
By the way, a conventional optical pickup uses a technique of dividing a returning light beam from a disk into a plurality of light beams. This division of the returning light beam is needed to obtain means for avoiding a tracking offset.
In cases where the tracking control is performed on a push-pull technique on condition that a beam spot is divided into several spot pieces on a detector, a tracking offset will be caused due to eccentricity of an objective lens. The light beam is divided into two light fluxes at a spot on the objective lens and the light fluxes are detected by respective detectors, so that no tracking offset will be caused.
In addition, when a DPD (Differential Phase Detection) tracking technique is used, it has been known that a tilt of a disk in its tangential direction causes a tracking offset. This kind of tracking offset can be removed by dividing a light beam into four light beams at a spot on an objective lens.
However, the aberration correcting means shown in FIG. 1B uses the two liquid crystal layers 4 and 5 which operate in the same manner. That is, the liquid crystal layers 4 and 5 are dedicated to only a limited operation that gives the same phase distribution to both of the outward and homeward light beams, as follows. Light irradiated from a light source enters the liquid crystal layer 5 in the form of parallel light beams, while the light returning from the optical disk DK2 enters the liquid crystal layer 6. As a result, the light that has passed the liquid crystal layers 5 and 6 is returned to parallel-beam light beams, which is the same as the incident light.
Hence, as means for dividing the returning light from the optical disk into plural light beams, a polarizing hologram has been used additionally.
However, such a manner is obliged to add one more optical part to the optical system in which the two liquid crystal panels have already been arranged. This has faced an obstacle that the pickup becomes larger in its size and higher in its production cost.